Methods for determining engine rough running by evaluating the engine rpm are used for spark-ignition engines as well as in diesel engines in order to detect a non-uniform running of the engine and to minimize this non-uniformity via suitable control arrangements. The non-uniform running of the engine can be caused, for example, by valve coking or, in direct injection engines, by scattering of the characteristic values of the injection valves.
Methods of this kind utilize the realization that a non-occurring or incomplete combustion within a cylinder of an engine is associated with characteristic changes of the torque trace of the engine compared to the normal operation.
From the comparison of the torque traces, one can distinguish between normal operation of the engine without misfires and so-called operation associated with misfires or non-optimal combustion. An incomplete or poor combustion of one or several cylinders contributes to a total torque trace of the engine with a reduced contribution. This contribution can be determined from a detection of the actual torques of the cylinders via an evaluation of the time-dependent trace of the crankshaft rotation or the camshaft rotation.
According to the method of the invention, a crankshaft angular region characterized as a segment is assigned to a specific region of the piston movement of each cylinder. The segments belonging to each cylinder are realized by markings on a transducer wheel coupled to the crankshaft. The segment time is the time in which the crankshaft passes through the corresponding angular region of the segment and is dependent essentially on the energy converted in the combustion stroke. Misfires or a poor combustion lead to an increase of the ignition synchronously detected segment times as a consequence of the deficient torque contribution. These segment times are determined for each cylinder by scanning the markings on the transducer wheel with a suitable sensor. The more uniform the engine runs, the smaller will be the differences between the segment times of the individual cylinders.
In accordance with methods already known from the state of the art, such as disclosed in German patent publication 4,138,765 (corresponding to U.S. patent application Ser. No. 07/818,884, filed Jan. 10, 1992, now abandoned), an index for the rough running of the engine is computed from differences of the segment times. Additional conditions such as the increase of the engine rpm for a vehicle acceleration are compensated by computation. The rough-running value, which is computed for each ignition, is compared in a subsequent method step in synchronism with the ignition in a desired value comparison to a threshold value. If the determined rough-running value exceeds the threshold value, which is dependent upon operating parameters such as load and rpm, then this is evaluated, for example, as a misfire of the particular cylinder.
In a further method step, equalization or correction factors are formed for individual cylinders in an evaluation unit and, with the aid of these factors, injection times, ignition time point times, or the charge of the individual cylinders, which are affected by the torque changes, can be influenced. For example, a change of the ignition time point can change the torque component of a cylinder. Furthermore, by influencing injection times and injection duration, the differences in the injection performance of injection valves can be compensated. Furthermore, for a system with cylinder-individual adjustment of the cylinder charge (for example, via individual throttle flaps or fully variable inlet and/or outlet valves), the cylinder individual charge can be adapted.
The above-described method, which is known from the state of the art, has been proven in the context of engine management systems for bringing about a cylinder equalization. The cylinder-individual interventions undertaken here can, however, lead to the situation that a knocking combustion in one or several cylinders, which occurs under specific conditions, is additionally amplified. For this reason, in known engine management systems, which have a knock control in addition to the cylinder equalization, a use of both systems at the same time is precluded. This can be attributed to the situation that the changes of the ignition angle, which are undertaken because of the knock control, can lead to a change of the rough-running value. These changes of the ignition angle are caused, for example, by an ignition angle retardation. Should the cylinder equalization by means of injection time correction be active in this case simultaneously with the knock control, then the cylinder equalization for reducing the rough-running values would effect an enrichment of the gas mixture because of changed injection times in the cylinder subjected to knocking combustion.
However, an enrichment of the above kind perforce leads to a further increase of the knocking combustion within the cylinder during the combustion so that neither the objective of a quiet engine running nor the elimination of the knocking combustion can be achieved by the cylinder equalization and the knock control of the engine management system.
From the state of the art, possibilities are known for acting on the operating state of internal combustion engines. In contrast thereto, the method of the invention affords the advantage that, for the first time, a knock control and a cylinder equalization can be combined without the concern that there will be a larger misidentification in the detection of misfires in the context of the cylinder equalization and, on the other hand, without the knock tendency of the particular engine being increased because of the measures of cylinder equalization. This takes place in accordance with the invention in that the equalization factors (that is, correction factors) for the change of the injection times or the ignition time points serve as the basis for the determination of a rough-running increase value effected by the change and in that, with the determined rough-running increase value, a computation of a corrected rough-running value takes place. The above-mentioned equalization factors and/or correction factors are computed in the context of the rough-running computation for the cylinder equalization. The corrected rough-running value is used for the computation of the final equalization factor and/or correction factor for influencing the injection times or the ignition time points.
Because of the fact that the rough-running value, which is decisive for the final computation of the equalization factors and/or correction factors, is subjected to a correction step in advance of the computation, the rough-running value increase, which takes place normally without correction, is eliminated so that an unwanted interaction between knock control and cylinder equalization is avoided.
The rough-running increase value, which is to be determined, can, according to the invention, be determined in different ways. Thus, it is conceivable to determine this value from the computed equalization factor and/or correction factor via a characteristic field computation. The relationship between the rough-running increase, which is to be determined, and an ignition angle shift, which is to be carried out, is stored in a characteristic line within the engine management system so that the rough-running increase can be determined in a simple manner. Furthermore, the possibility is present to determine the rough-running increase value from the torque as a function of the ignition angle efficiency. This affords the advantage that the relationship between torque and ignition angle efficiency is already stored in the engine system as a function. Furthermore, it is conceivable to determine the rough-running increase value as a function of the injection time span in the expansion phase of the particular cylinder (so-called double injection). This applies also for the individual charge of a cylinder. Here too, the charge difference to the other cylinders can be considered via the rough-running increase value.